The adsorption of silicate and phosphate from pure and mixed solutions by four soils known to fix phosphate strongly has been measured.The adsorption at constant pH for each acid from its pure solution follows the Langmuir isotherm. The maximum adsorption of silicate by three of the soils from the pure solution occurs at pH 9.2, and the maximum adsorption curve for phosphate by two of the soils from its pure solution has a break in its slope at pH 6.4 and 11.6. These pHs are all just below the pK values for the dissociation of hydrogen ions from the undissociated silicic acid or the acid phosphate anions. The presence of silicate in a mixed solution, which is sufficiently concentrated in both silicate and phosphate to give maximum adsorption of either if in pure solution, does not affect the amount of phosphate adsorbed until the pH is over 6.5-7.0. At this pH the maximum adsorption curve for silicate crosses that for phosphate. The presence of phosphate in the mixed solution always depresses the adsorption of silicate. The maximum amount of silicateplus-phosphate adsorbed from this mixed solution is either a little less than or equal to the amount of silicate adsorbed from the pure silicate solution if the pH is above 7. When silicate displaces phosphate, or phosphate displaces silicate, more moles of the displacing acid are adsorbed than moles of the displaced acid released.These results for soils are similar to those of Hingston et al. for the adsorption of silicate and phosphate by goethite.